Copper

Copper conducts electricity at a rate 97% that of silver, and is the standard for electrical conductivity. Copper provides a diverse range of properties: good thermal and electrical conductivity, corrosion resistance, ease of forming, ease of joining, and color. In addition, however, copper and its alloys have relatively low strength-to-weight ratios and low strengths at elevated temperatures. Some copper alloys are also susceptible to stress-corrosion cracking unless they are stress relieved.

Copper and its alloys — the brasses and bronzes — are available in rod, plate, strip, sheet, tube shapes, forgings, wire, and castings. These metals are grouped according to composition into several general categories: coppers, high-copper alloys, brasses, leaded brasses, bronzes, aluminum bronzes, silicon bronzes, copper nickels, and nickel silvers.

Copper-based alloys form adherent films that are relatively impervious to corrosion and that protect the base metal from further attack. Certain alloy systems darken rapidly from brown to black in air. Under most outdoor conditions, however, copper surfaces develop a blue-green patina. Lacquer coatings can be applied to retain the original alloy color. An acrylic coating with benzotriazole as an additive lasts several years under most outdoor, abrasion-free conditions.

Although they work harden, copper and its alloys can be hot or cold worked. Ductility can be restored by annealing or heating incident to welding or brazing operations. For applications requiring maximum electrical conductivity, the most widely used copper is C11000, “tough pitch,” which contains approximately 0.03% oxygen and a minimum of 99.0% copper. In addition to high electrical conductivity, oxygen-free grades C10100 and C10200 provide immunity to embrittlement at high temperature. The addition of phosphorus produces grade C12200 — the standard water-tube copper.

High-copper alloys contain small amounts of alloying elements that improve strength with some loss in electrical conductivity. In amounts of 1%, for example, cadmium increases strength by 50%, with a loss in conductivity to 85%. Small amounts of cadmium raise the softening temperature in alloy C11600, which is used widely for printed circuits. Tellurium or sulfur, present in small amounts in Grades C14500 and C14700, has been shown to increase machinability.

Copper alloys do not have a sharply defined yield point, so yield strength is reported either as 0.5% extension under load, or as 0.2% offset. On the most common basis (0.5% extension), yield strength of annealed material is approximately one-third the tensile strength. As the material is cold worked or hardened, it becomes less ductile, and yield strength approaches tensile strength.

Copper is specified according to temper, which is established by cold working or annealing. Typical levels are: soft, half-hard, hard, spring, and extra-spring. Yield strength of a hard-temper copper is approximately two-thirds of tensile strength.

For brasses, phosphor bronzes, or other commonly cold-worked grades, the hardest available tempers are also the strongest and represent approximately 70% reduction in area. Ductility is sacrificed, of course, to gain strength. Copper-beryllium alloys can be precipitation hardened to the highest strength levels attainable in copper-base alloys.

The ASME Boiler and Pressure Vessel Code should be used for designing critical copper-alloy parts for service at elevated temperatures. The code recommends that, for a specific service temperature, the maximum allowable design stress should be the lowest of these values as tabulated by the code: one-fourth of the ultimate tensile strength, two-thirds of the yield strength, and two-thirds of the average creep strength or stress-rupture strength under specified conditions. Silicon bronzes, aluminum brasses, and copper nickels are widely used for elevated-temperature applications.

All copper alloys resist corrosion by fresh water and steam. Copper nickels, aluminum brass, and aluminum bronzes provide superior resistance to saltwater corrosion. Copper alloys have high resistance to alkalies and organic acids, but have poor resistance to inorganic acids. One corrosive situation encountered, particularly in the high-zinc alloy, is dezincification. The brass dissolves as an alloy, but the copper constituent redeposits as a porous, spongy metal. Meanwhile, the zinc component is carried away by the atmosphere or deposited on the surface as an insoluble compound.

Designating alloys: Originally developed as a three-digit system by the U.S. copper and brass industry, the designation system for copper-based alloys has been expanded to five digits preceded by the letter C as part of the Unified Numbering System for Metals and Alloys (UNS). The UNS designations are simply an expansion of the former designation numbers. For example, Copper Alloy No. 377 (forging brass) becomes C37700. Numbers C10000 through C79900 are assigned to wrought compositions, and numbers C80000 through C99900 to casting alloys.

The designation system is not a specification; rather, it is a method for identifying and defining the chemical composition of mill and foundry products. The precise requirements to be satisfied by a material and the temper nomenclature that applies are defined by the relevant standard specifications (ASTM, Federal, and Military) for each composition.

There are approximately 370 commercial copper and copper-alloy compositions. Brass mills make wrought compositions in the form of rod, plate, sheet, strip, tube, pipe, extrusions, foil, forgings, and wire. Foundries supply castings. The following general categories apply to both wrought and cast compositions.

Coppers, high-copper alloys: Both wrought and cast compositions have a designated minimum copper content and may include other elements or additions for special properties.